Some touch screen device types, such as touch enabled computer monitors, personal digital assistants, media players, and cellular telephones, enable users to interact with onscreen elements by touching a display with a finger or a stylus. Single-touch devices are capable of recognizing a single point of contact, and they enable users to perform a variety of two-dimensional (2D) interactions such as moving on-screen objects through a so-called “dragging” operation.
More recently, devices have emerged that recognize more than one simultaneous point of contact for interacting with 2D on-screen objects. Such so-called “multi-touch” interfaces generally support single-touch interactions, as well as multiple-touch (generally two-fingered) gestures. These multi-touch gestures are sometimes known as rotate-scale-translate (RST) interactions because these interactions generally allow a user to spin, slide, or resize 2D objects in a 2D plane. One commonly used multi-touch gesture involves a user touching a 2D object at two or more points on a display and then revolving those points relative to each other to effect virtual rotation of the object, similar to the manner in which a person could spin a photograph or a piece of paper lying on a tabletop. Another commonly used multi-touch gesture involves a user touching a 2D object at two points on a display, and then spreading or pinching the gap between the two points to enlarge or shrink the visible size of the object.